Presently available infrared radiation detector devices have numerous physical limitations and electrical limitations. The prior art devices make use of a photoresistor commonly called a photoconductor circuit. In this device, a semiconductor element is placed in series with a resistance load and a current is produced by voltage. The resistance of the semiconductor substrate changes according to the infrared radiation which the semiconductor is exposed to so that the current flowing through the semiconductor and the load resistor varies. It has been a practice to use these infrared detectors in arrays of 180 or less. It has been extremely difficult to use arrays greater than 180 because of the physical limitations. For example, there is power dissipated in the semiconductor according to the joule heating effect of P=i.sup.2 R. In order for these devices to operate, the detector must be kept below 90.degree. Kelvin. When a large array of these devices is built they create a great load on the cooler and make it extremely difficult to transfer the heat. Another great disadvantage of photoresistor devices is that the geometry presents physical limitations beyond the ability to etch the substrate. The third problem in these devices is the GR noise component. The generating noise comes from movement of electrons from the valence band to the conduction band which is at random. The recombining noise is also random noise independent of the generation noise as electrons move from the conduction band to the valence band. It is highly desirable to find additional methods of providing infrared detectors.
The present invention uses a metal-insulator-semiconductor (MIS) detector for infrared radiation. An MIS device does not have a current flow. Therefore a major problem in the photoresistor devices is overcome Additionally, the heat load on the cooler is considerably reduced because the lack of the current flow and the power is determined according to the formula P=1/2CV.sup.2. In an MIS device the C is extremely small so that the power is negligible. An additional advantage of the MIS device is that a voltage is taken after an amount of charge has been integrated over a set time so that there is no recombination noise This feature alone cuts the noise in half so that the only noise is generation noise.
The following publications may be of interest in the same field as this invention. Encyclopedia of Chemical Technology, Volume 17, Third Edition Copyright @1982 by John Wiley & Sons, Pages 601-611. This describes an MIS devices and provides equations for MIS devices. This publication was written by one of the inventors herein and is hereby incorporated by reference into this application. A second publication which describes use of a ramp voltage for potential wells is entitled "Increased charge capacity in breakdown-limited metal-insulated-semiconductor HgCdTe devices using a ramped gate voltage", published in Applied Physics Letters Volume 37, Number 4, 15 Aug. 1980 and copyright by the American Institute of Physics, also incorporated by reference into this application.
To this point there has been extreme difficulty in producing an operable MIS infrared radiation detection device. The use of an MIS device presents an additional problems which must be solved before an MIS device can be useful as an infrared radiation detector. It is desirable to measure infrared radiation having a wavelength between the range of 3 to 5 or 8 to 12 microns because this is the wavelength of infrared radiation which is of particular interest.
The peak power of infrared radiation at 300.degree. Kelvin has a wavelength of 10 microns. This presents the problem that an infrared radiation detector operating in this range will quickly be saturated because many surrounding objects will be at 300.degree. Kelvin It is therefore desirable to measure all the desired infrared radiation at this temperature and not saturate the MIS device. The photon density at this temperature range is extremely large, generally in excess of 5 .times.10.sup.17 photon/cm.sup.2 -sec-steradians. A photon density of this magnitude quickly saturates known MIS devices because of the high density.
The present invention uses the MIS device as a photo capacitor. A metal plate is placed above the bulk, that is, substrate, with an insulator in between. When a pulse voltage is applied to the metal plate, a potential well is created in the bulk material below. This potential well represents, for n type semiconductor, the pushing of the electrons back out of that portion of the substrate and the ability of the photons to generate holes which will be attracted to the surface of the substrate and appears as voltage as the charges are collected. When a large number of holes have migrated to the surface, the device becomes saturated as represented by the well being full. At this point additional photons will not generate any additional charge in the potential well.
A solution in the past has been to attempt to increase the pulse voltage so that the well potential is greater and can hold additional charge to prevent the device from becoming saturated in a very short period of time. If the substrate being used is HgCdTe with a small band gap energy as in the present invention, an increase of voltage greater than 1 voltage as the pulse volt causes tunnelling of the electrons from the valence band and creates additional holes from the tunnelling effect which do not result from photons. The tunnelling decreases the potential of the well. The substrate of HgCdTe or other narrow bandgap semiconductors is particularly sensitive to tunnelling if the applied pulse voltage is excessive.
The timing of an MIS device is also critical. A pulse voltage must be quickly applied to the metal layer and then the voltage of the metal layer must be allowed to drift to follow the amount of charge which is built up in the potential well. Just prior to the well being saturated, the voltage difference between the semiconductor and the metal layer must be determined in order to determine the amount of infrared radiation which the substrate was exposed to. This can be an extremely short period of time and the high density of infrared radiation in the wavelength being measured causes saturation of the potential well so quickly that it is extremely difficult to measure the voltage difference after the pulse has been applied as the voltage of the metal layer is allowed to float.
The present invention solves many of the problems associated with known MIS infrared radiation detection devices. The present invention provides for the creation of a potential well of an area which is much larger than the area through which photons are allowed to pass to the substrate below. This has the effect of decreasing the photon density for the potential well in the substrate below the gate. This creates an infrared radiation detection region coupled to a charge storage region which can store all the charges as they are generated within the potential well. The detection gate, that is, the transparent gate, is kept small with respect to the overall gate area because of the high density of infrared radiation at the desired wavelength and the sensitivity of the substrate to this radiation.
In one embodiment, the entire gate is a single piece of metal with the storage gate being much thicker than the transparent gate over the infrared radiation sensitive region. This results in the entire gate being at the same voltage at all times and the potential well being equally great in the substrate beneath the entire length of the gate. A pulse voltage can be applied to the gate and then the voltage of the gate allowed to float as the device is exposed to infrared radiation After a certain period of time which will be determined as a time less than the saturation of the now extremely large storage well, the difference in voltage between the gate and the substrate is determined which will be proportional to the amount of infrared radiation which the device is exposed to. The gate may be isolated on either side by a channel stop which prevents the potential well from being created in undesired regions in the substrate or may be isolated by other methods well known.
In an alternative embodiment, the transparent gate is a separate metal strip from the storage gate. A transfer gate is placed in between the two gates to form a three gate MIS device. In this embodiment, the detector gate, which is of extremely thin metal, is pulsed with the voltage or a ramp voltage is applied to create a potential well in the substrate. As the potential well begins to fill up, the transfer gate turns on to transfer the charge to the storage region prior to the potential well under the detection gate being saturated. After the change is stored in the storage area, the amount of charge can be read by determining the voltage difference between the thick metal layer and the substrate below which represents the storage area. This embodiment is particularly useful because a ramp voltage can be applied to the detector gate or, in the alternative, constant voltage can be applied to the detector gate during the exposing of infrared radiation There is no requirement for the detector gate to have a voltage which will float because the voltage of the detector gate will not be measured. The detector gate can be subjected to a fixed voltage independent of the storage gate whose voltage must be allowed to float to determine the amount of charge stored in the charge storage region This is particularly useful if a ramp voltage is applied to the detector gate.
It has been found that the potential well can be greatly increased if a ramp voltage of a specific slope and frequency is applied to the metal layer representing the detector gate above the substrate This serves to deepen the well to a considerable extent, many more times than is possible if a pulse is used due to the physical characteristics of narrow band gap semiconductors, such as, HgCdTe. This allows an extremely large potential well to be built up with a relatively small area being used as the detection gate. As this deep potential well is filled, the ramp voltage steadily increases the voltage so that the potential well becomes deeper to prevent saturation before the desired time. As the ramp voltage reaches a peak, the transfer gate transfers the entire charge to the storage region whose voltage can be allowed to float and is not required to follow the ramp voltage which was applied to the detector gate. The charge as stored in the charge region underneath the storage gate will cause the voltage difference between the metal layer representing the storage gate and the substrate immediately below to vary in proportion to the amount of charge. This will be proportional to the amount of infrared radiation which the detector gate was exposed to.
An alternative embodiment is to have several detection regions connected to feed charge into a single storage region. A large storage region can be used to multiplex a desired number of detection gates together. This embodiment would have a plurality of detection regions connected by transfer gates to a single storage region whose charge could be read. The charge could thus be made to represent any one of several detection regions or any combination of detection regions. In this embodiment, the storage area can act as a multiplexer to select a detection region Alternatively, the storage area can be a large storage for many detection regions added together.
This invention overcomes considerable disadvantages in the prior art in the use of MIS device; as an infrared radiation detector.
It is an object of this invention to provide an MIS device which has improved characteristics for use as an infrared radiation detector.
It is an object of this invention to provide an improved method for detecting infrared radiation using an MIS device.
It is a further object of this invention to provide a means for transferring charge generated in a potential well, which was exposed to infrared radiation, to a storage area whose voltage is not tied to the voltage of the detector gate.